I. Field of the Invention
The present invention relates generally to transparent armor constructions.
II. Description of Relevant Art
Transparent armor has long been used for armored vehicles in both military and nonmilitary applications. Such transparent armor is typically used for the windows of such vehicles and thus must be transparent or at least substantially transparent.
Oftentimes, the previously known transparent armor is a laminate structure of ceramics, glasses, resins, and polymers in an asymmetric design that is sufficient to meet ballistics requirements, i.e. the ability to protect personnel from such ballistics. While these previously known laminate structures for transparent armor have functioned adequately to protect personnel from ballistics, there have been a number of previously known issues and problems associated with such transparent armor.
Perhaps of most importance, is that the previously known transparent armor suffers both degradation and failure due in large part to the residual thermal stresses that are present in such asymmetric designs when subjected to extreme temperatures. Such degradation and failure includes fractures, loss of dimensional tolerances due to bending of the transparent armor, delamination, fogging, and debonding. When this occurs, the windows must be replaced at significant cost.
A primary source of degradation and failure of the transparent armor arises from the different materials of the layers that form the laminate which have different coefficients of thermal expansion. These laminates of dissimilar materials having different coefficients of thermal expansion have effectively limited the temperature range for the transparent armor or windows before a failure may occur. For example, typically such failure occurs at temperatures of about −30° Centigrade or warmer for both lightweight transparent armor as well as transparent armor for heavy-duty vehicles. However, many vehicles, for example military vehicles, must be able to operate at temperatures below −30° Centigrade without failure.
In addition, bonding of two dissimilar materials having a high differential coefficient of thermal expansion, i.e. bonding glass to polymer, itself induces a thermal stress in the resulting laminate. These stresses can cause the premature failure from fracture, debonding, or deformation of the laminate.
In order to limit the stress caused by a high differential of the coefficient of thermal expansion between adjacent laminate layers, it has been previously known to mitigate the stresses by using compliant bonding agents as well as lower temperature processing methods to produce the laminate. However, in many circumstances, these mitigation techniques are only marginally successful. Indeed, where the laminate includes layers of both glass and ceramic/plastic, the differential coefficient of thermal expansion is still high and the processing temperatures also high that failure of the resulting laminate transparent armor is common.